NoMa station was a backfill station that opened in November 2004. As you (may) know the station is built on existing above ground tracks; why put it underground just to have to go right back up when the next stop (Rhode Island) is above ground? Plus, the Amtrak tracks are already above ground and so there's already a RR bridge there, so why bother?

There was MUCH debate about where to put the silver line (above/below). One of my favorite things ever was the public testimony from people who argued that there was no cost increase to going underground. They said that in public with a straight face!